1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices having a titanium silicide, and more particularly, to a method of forming a polycide structure of which a component thereof is a titanium silicide.
2. Description of the Related Art
Advances in the miniaturization of semiconductor devices have been made every year, so that MOS transistors in which a gate length is, for example, 1 .mu.m or less are available on the market. However, the resulting increase in the IC complexity causes media delay of the gate employing polysilicon, and thus there has been a demand for a gate material having a low resistivity.
To meet this demand, a gate electrode having a polycide structure, in which a tungsten silicide or a molybdenum silicide is disposed in an upper portion thereof while a polysilicon is disposed in a lower portion thereof, has been developed. The sheet resistance of the polysilicon is 20 .OMEGA./.quadrature., the sheet resistance of molybdenum polycide is 5 .OMEGA./.quadrature., and the sheet resistance of tungsten polycide is 2 .OMEGA./.quadrature..
Among the above-described gate materials, polysilicon was put into practical use for the first time. Molybdenum polycide followed it, which was followed by tungsten polycide. So far, the practical use of a gate material having a lower resistivity than tungsten polycide has not been realized.
Under the above-described circumstances, titanium polycide is known as a material having a lower resistance than tungsten polycide. Titanium polycide can offer a sheet resistance of 0.5 .OMEGA./.quadrature. through 1.0 .OMEGA./.quadrature., which is the lowest resistance ever offered by gate materials having a polycide structure. Conventionally, titanium polycide is formed by a method called the alloying method. FIG. 5 illustrates the concept of this alloying method.
First, a polysilicon 203 and a pure metal titanium 204 are deposited on an insulating film 202 of, for example, silicon dioxide. Low pressure CVD is generally used as the deposition method for the polysilicon 203, and the pure metal titanium 204 is generally formed by sputtering (FIG. 5(a)).
Next, heat treatment is conducted at about 800.degree. C. in a gas which is inactive to the pure metal titanium, such as a forming gas, to cause the silicide formation reaction of the upper titanium 204 and the lower silicon 203 to take place. This results in formation of a titanium silicide 205 on top of the silicon 203 (FIG. 5(b)).
The titanium polycide formed in the above-described manufacturing method exhibits a very low resistance. However, application of such a titanium polycide to the manufacturing process of semiconductor devices has following two crucial drawbacks.
The first drawback is that hydrofluoric acid cannot be used to wash the titanium prior to the heat treatment required for silicide formation because titanium is soluble in hydrofluoric acid, and will be dissolved instantaneously. Since the heat treatment required for forming silicide is conducted at a relatively high temperature of about 800.degree. C., if washing with hydrofluoric acid is impossible, the impurities attached to the surface of the wafer diffuse into the semiconductor substrate, thus increasing the possibility of device breakdown. This is the primary reason why the polycide having the titanium silicide on the top thereof is not put into practical use.
The second drawback is that an inactive gas, such as a forming gas, must be used in the heat treatment to form the silicide. This is because titanium is a very active element, which can be oxidized in the presence of a very small amount of oxygen or can be nitridized in the presence of nitrogen. Accordingly, a forming gas is generally used. However, a forming gas is very expensive as compared with nitrogen, thus increasing production cost. This is the second drawback of the alloying method.